The present invention relates to a rolling bearing, and more particularly to a rolling bearing for use in industrial machines and the like under high-temperature conditions, typically 130.degree. C. or more.
Rolling bearings are generally used under severe conditions such that they are subjected to repeated shearing stress under high contact pressure. Therefore, the rolling bearings are required to have sufficiently high rolling fatigue life (hereinafter referred to simply as "life") so that they can withstand the applied shearing stress. A bearing material for rolling members such as an inner race, an outer race, rolling elements and the like forming the rolling bearings has conventionally been provided with high carbon chromium steel, which is subjected to hardening and tempering at 160.degree. to 200.degree. C. to provide a Rockwell hardness of H.sub.R C 60 to 64 to thereby achieve a longer life.
When such rolling bearings are used under high-temperature conditions at 130.degree. C. or more, the retained austenite phase existing in the high-carbon chromium steel as the bearing material is decomposed. Accordingly, deterioration of dimensional stability occurs and the rolling members expand within a short time.
Under these circumstances, it has been proposed that rolling bearings intended for use at high temperatures should be subjected to tempering at a temperature, e.g. 240.degree. C. or more, that is higher than the above-described usual tempering temperature (160.degree. to 200.degree. C.). Thus, the retained austenite is completely decomposed so that its content becomes substantially 0 wt % before finished products of rolling bearings are manufactured (this approach is hereinafter referred to as the "first conventional method").
Since the first conventional method involves tempering at a high temperature, the produced rolling bearing has a reduced hardness and thus a shortened life. On the other hand, since the dimensional stability is superior, the rolling bearing is acceptable to some extent.
Another approach has been proposed in which a high-carbon chromium steel includes Cr, Mo, V, Si and other elements that have a superior resisting property for temper softening (for example, Unexamined Japanese Patent Publication No. Hei. 3-82736; this approach is hereinafter referred to as the "second conventional method").
In the second conventional method, since elements such as Cr, Mo, V and Si which have the high resisting property for temper softening are added to the high-carbon chromium steel, the reduction in bearing's hardness can be suppressed to some extent even if tempering is performed at high temperatures of 240.degree. C. or more.
According to still another approach, a high-carbon steel material is subjected to carbonitriding or nitriding for the purpose of increasing the surface carbon or nitrogen contents to form carbonitrides or nitrides. Subsequently, the steel is subjected to hardening and tempering at a high temperature of 200.degree. to 250.degree. C. As a result, the retained austenite remains in the surface while the retained austenite existing in the core is adjusted to be substantially 0 wt % (for example, examined Japanese Patent Publication No. Hei. 3-56305; this approach is hereinafter referred to as the "third conventional method").
In the third conventional method, since tempering is performed at a temperature of 200.degree. to 250.degree. C. where the retained austenite in the core of rolling members becomes 0 wt %, the dimensional stability is improved. In addition, a large amount of the retained austenite and carbonitrides are precipitated in the surface in order to extend the life of the rolling bearing.
However, the three conventional methods have the following problems.
In the first conventional method, the superior dimensional stability has been assured at the sacrifice of life characteristics. However, with the recent tendency of machines toward higher-speed operation and smaller size, the deterioration in the life characteristics has gradually become unacceptable.
In the second conventional method, the elements such as Cr, Mo, V and Si which have the superior resisting property for temper softening are added to a high carbon chromium steel so that it can suppress the reduce in hardness due to the tempering at high temperatures. However, the addition of these elements increases the cost of materials. Further, the addition of more alloy elements reduces the productivity to thereby increase the overall production cost.
In the third conventional method, the retained austenire in the core of rolling members is adjusted to be substantially 0 wt %, so that the dimensional stability of the bearing is improved. In addition, the content of carbon C contained in the high carbon steel material itself and the content of nitrogen N are sufficiently increased to precipitate carbonitrides while a large amount of retained austenite is precipitated in the surface of the bearing to increase not only its surface hardness but also the life of the bearing. However, there is a problem with this approach in the case where the depth of carbon and nitrogen diffusion is excessive with respect to the thickness of the bearing, namely, in the case where the core of the bearing is unduly limited in volume, the core signifying a range where the content of retained austenite is substantially 0 wt %. The reason is because the retained austenite in the surface layer portion decomposes to cause dimensional changes during the operation of the bearing at high temperature. Accordingly, the rolling bearing produced by the third conventional method suffers the problems that if it is used under high-temperature conditions, the retained austenite existing in the surface layer portion decomposes to cause the dimensions of the bearing to expand in a short time.